Lyn is the predominant Src family kinase in B cells and has a unique role in the feedback regulation of BCR signaling. Lyn knockout mice have hyperactive B cells and a phenotype resembling the human condition, systemic lupus erythematosus (SLE) with severe kidney disease. The goals of the proposed research are to identify the physical site of inhibitory signaling mediated by Lyn in B cells, and to use gene therapy to ameliorate the autoimmune kidney disease in Lyn-deficient mice. Lyn will be targeted to the lipid raft or non-raft compartment of the plasma membrane by engineering appropriate chimeric constructs. The chimeric proteins will be tested for their ability to reconstitute signaling by ectopic expression in Lyn-deficient B cells in vitro. The contribution of lipid raft localization of Lyn towards development of autoimmune disease will be tested in vivo by creating knock-in mice expressing raft-restricted or raft-excluded Lyn instead of wild type Lyn. Finally, an attempt will be made to ameliorate the lupus-nephritis phenotype of Lyn-deficient mice by repopulating them with autologous hematopoeitic progenitors reconstituted with wild type Lyn. I have extensive experience in studying immune cell signaling with emphasis on subcellular localization of signaling intermediates during B cell activation. The proposed research will expand my in vitro signaling and cell biology expertise to the study of in vivo mouse models of human autoimmune disease and retrovirus-mediated stem cell transplantation as a means of gene therapy. I will collaborate with investigators within and outside UCSF to execute the research plan. The proposed studies will generate critical molecular and cellular tools to establish a strong research program that aims to study the immune cell defect in SLE-like kidney disease and use gene therapy as a means of intervention. Ultimately, this project will enable me to develop into an independent investigator and pursue a tenure-track faculty position